As Savin pointed out right in the primary design doc (and Tobs created an entire topic to clarify) there are Reasons that companions were limited to one-per established writer: It's a huge undertaking since they need to be a participant (at least potentially) in every major part of the game, interact at various points with all the other companions and have enough content in both gameplay and writing terms to justify their constant presence. Or to put it more succinctly, 'no bubble waifus'.
Since everyone eligible but Savin (and the unique case of B) only gets one character, that naturally restricts them to whichever character they feel the most passionate about, or came up with first, or whatever else contributed to why they picked this character over that character. I'm sure that in some other hypothetical leg of the Trousers of Time we could have gotten a different set of companions but that's not the world we live in.